1. Field of the Invention
This invention relates in general to a method of making a semiconductor device. More particularly, the invention relates to a process for making a heterojunction avalanche photodetector using non-lattice matched semiconductors.
2. Description of Related Art
The use of semiconductor materials to create various electronic devices is largely dependent on the requirements of the device for a given task, the ability to use certain materials together in a given device, and the cost for the finished device. As device requirements are tightened or increased, new methods and materials combinations are required to meet the requirements for the device.
An avalanche photodetector (APD) has two functions: the absorption and conversion of light to an electrical signal, and the amplification of that electrical signal through avalanche multiplication. These functions can be done by a single material, such as silicon, or by two materials grown epitaxially, one for the absorption and another for the multiplication. The performance of an APD is based on the achievable signal processing speed and noise, which are dependent on the absorption and multiplication efficiencies. These parameters are expressed by the responsivity, the 3-dB frequency bandwidth, and the excess noise factor. The excess noise factor and 3-dB bandwidth are dependent on the total device thickness and the ratio between electron and hole ionization coefficients of the material used for multiplication. The larger the ratio between the electron and hole ionization coefficients, the larger the gain bandwidth product of the APD will be. Further, the larger the coefficient ratio, the less noisy the APD will be.
Current devices that have tried to maximize detector performance have fallen short of desired efficiencies due to the trade off between absorption coefficient and electron/hole ionization coefficients. Materials, such as silicon, that have high electron/hole ionization coefficient ratios do not have good absorption in the desired optical regions, such as the telecommunications wavelengths of 1.3 and 1.5 .mu.m. Materials that have good absorption do not have a high ionization coefficient ratio. Heterojunction devices have, until now, been limited to lattice matched materials, and device efficiencies have not been significantly increased through the use of heterojunction APDs because of the lattice matching limitation.
It can be seen then that there is a need for a method of making an APD that has high efficiency. It can also be seen that there is a need for a method of making an APD that has a high electron to hole ionization ratio in the multiplication region and a high absorption region for converting light into electricity. It can also be seen that there is a need for a device that can absorb light in the desired optical regions and efficiently and precisely convert that light into electrical signals.